1. Field of the Invention
The present invention relates to semiconductor optical waveguide switches and, more particularly, to high carrier injection optical waveguide switches.
2. Background Information
The two-mode interference (TMI) optical waveguide switch is one of the most important guided-wave devices in integrated optics. Several methods have been proposed for realizing such TMI optical waveguide switches, such methods being described for example in: Janz et al. xe2x80x9cMach-Zehnder switch using an ultra-compact directional coupler in a strongly-confining rib structure,xe2x80x9d IEEE Photon. Technol. Lett. Vol. 6, pp. 981-983, 1994; Gao et al. xe2x80x9cSiGe/Si bifurcation optical active switch based on plasma dispersion effect,xe2x80x9d Electron. Lett. Vol. 31, pp. 1740-1741, 1995; and Zhao et al. xe2x80x9cZero-gap directional coupler switch integrated into a silicon-on insulator for 1.3-xcexcm operation,xe2x80x9d Opt. Lett. Vol. 21, pp. 1664-1665, 1996. However, the optical waveguide switches described in these publications have only a single carrier injection region, resulting in a large injection current and injection current density when the optical waveguide switch is in operation.
According to conventional types of carrier injection, optical waveguide switches can be classified as either vertical injection or lateral injection. For a vertical injection type optical waveguide switch, one electrode is located on the top of the switch, the other electrode is located at the bottom of substrate, and the substrate must be n+ or p+ type for good ohmic contact. For either n+ type or p+ type substrate, a large amount of free carriers will be present at the interface between the substrate and the waveguide layer. Hence, carrier absorption loss will be very large at the interface when the switch is in operation. However, the injection current and injection current density cannot be reduced by any other means. With lateral injection, the carrier absorption loss at the interface between the substrate and the waveguide layer can be eliminated due to the avoidance of either n+ or p+ substrate. However the injection current and injection current density still cannot be reduced.
FIG. 1 is a schematic plan view showing a conventional example of the lateral injection TMI optical waveguide switch. In FIG. 1, two optical waveguides, 1 and 2, cross at an angle xcex8 to form a Y-branch and function as the inputs of the switch, while optical waveguides, 3 and 4, cross at an angle xcex8 to form another Y-branch and function as the outputs of the switch. Waveguide 5 is a TMI section and serves as a refractive index modulation region. Reference numbers 6 and 7 refer to the carrier injection region and carrier collection region, respectively.
FIG. 2 shows a sectional structure view of the optical waveguides 1, 2, 3 and 4, which is taken along line Ixe2x80x94I of FIG. 1. In the structure of FIG. 2, reference number 8 is a substrate, 9 is a buffer layer, and 10 is a core waveguide layer. The rib-shaped optical waveguide is formed by reactive ion etching. The whole structure is covered by an insulating film 11. The buffer layer 9 is used to avoid scattering loss due to impurities and the absorption loss of carriers which are located at the surface of the substrate.
FIG. 3 is a sectional structure view of the middle section of the optical waveguide switch, which is taken along the line IIxe2x80x94II of FIG. 1. In the structure, reference numbers 8, 9, 10 and 11 refer to the same elements as in FIG. 2. The rib-shaped optical waveguide is formed by reactive ion etching. Two ion implantation regions 12 and 14 are formed in the upper layer 10. An insulating film 11 of SiO2 and two metal electrodes 13 and 15 are evaporated and formed.
In the above-described lateral structures, the substrate used can be lightly n or p doped, and the carrier absorption loss from the use of n+ or p+ substrate can be avoided. However, in order to achieve the switching from the output port 4 to 3, a large applied bias voltage, i.e., a large injection current, is still required.
An object of the invention is to provide a high carrier injection optical waveguide switch capable of reducing the switching injection current and injection current density at the time of switching. It is a further object of the present invention to provide a high carrier injection optical waveguide that, in addition to low power consumption, has low optical loss.
These and other objects of the invention are achieved by providing a higher carrier injection optical waveguide that includes: a pair of optical waveguide elements, one functioning as an optical waveguide input and the other functioning as an optical waveguide output; a TMI region, made of semiconductor material, between the optical waveguide input and the optical waveguide output; first and second carrier injection regions; and a lateral carrier collection region, the lateral carrier collection region and the first carrier injection region being positioned on opposite sides of the TMI region with the second carrier injection region being positioned between the lateral carrier collection region and the first carrier injection region. According to one implementation of the present invention, the input and output optical waveguides are each configured as a Y-branch connection of two single-mode rib waveguides. These single-mode waveguides include a waveguide layer on a substrate with a buffer layer. The width of the single-mode waveguides is w. The TMI region includes a waveguide layer on a substrate with a buffer layer. The width of the TMI region of the switch is 2 times that of the single-mode waveguide. On the top surface of the TMI region and beside the TMI region, two carrier injection junctions are made to inject the carriers into the refractive index modulation region, i.e., the TMI region, when they are forward biased.
In one implementation of the present invention, the high carrier injection optical waveguide switch is fabricated using Si-based SiGe material and standard Si technology. Thus, the present invention provides a high carrier optical waveguide switch that is simple to fabricate and easy to operate. Hence, the present invention is very suitable for silicon-based monolithic and hybrid optoelectronic integration.